The invention relates to a magnetic field generating assembly for use in nuclear magnetic resonance (NMR) apparatus.
Conventional NMR apparatus has a large solenoidal magnet which generates a high strength magnetic field within its bore with, at the centre of the bore, a working region in which the magnetic field strength is substantially uniform. To achieve the high strengths required for NMR imaging experiments, superconducting solenoids must be used with the superconducting material housed in large cryostats. Additionally, in order to perform an experiment a gradient coil system is provided, usually formed by resistive conductors, to superimpose magnetic gradients on the field within the working region. In operation, a sample to be examined is inserted into the bore of the magnet so that the region of the sample to be examined is located within the uniform working region and the experiment then performed. In order to allow maximum space for the sample, the gradient field system is positioned close to the inner surface of the main coils and this leads to a number of disadvantages. In particular, there is significant coupling between the gradient coils and the surrounding magnetic structure. For example, whenever the current in a gradient coil is changed, the flux threading conductive parts of the magnet structure changes so inducing an emf and an opposing eddy current. This reduces the speed at which each gradient change can be achieved and thus the speed of the overall experiment. Furthermore, large currents need to be supplied to the gradient coils in order to achieve useful magnetic field gradients and changes in these currents also lead to large eddy currents.